It's solely a bathroom, and not part of a guest house or the garage or anything? That's pretty strange these days.
The only thing I can think of is that maybe they have a pool, and it's there to shower off and change or use the toilet and sink without running inside all wet. Usually there'd be some sort of cabana/poolhouse or something rather than just a bathroom, though.
Well I, for one, am glad this has been resolved to your satisfaction. I can only imagine how it would have escalated after the Facebook wall post - stealing his towel while he's in the shower, flushing the toilet multiple times so he'd get scalded by the surge of hot water, changing his shampoo for glue....
Someone's been watching Police Academy 2 this week.... These days, revenge must include a webcam, otherwise it didn't happen.